Greetings,
I make tampers also.
Some of the things said already, aesthetics, personal import, finding one wherever you sit down, etc. are all important and about the owner.
Also about the owner, what are the functions that you want from your tamper? Some peaple what only to tamp - a flat-ended dowel does it. Some want to pick, poke, dig, take it apart. or play withe it in some other fashion. You know who you are.
As to function, I think that there are two fairly important qualities.
There should be some significant portion of the stamping end which is round. The shape of the bowl is round, the item tamping inside of it should be round.
Second, the length of the post which enters the bowl while tamping should be no greater in diameter than the stamping end.
There are some smaller tweeks which can be nice, like slanting the end of the foot so that it touches the ash at the edge of the bowl before the center.
Just opinions,
anthony